El Niño diversity during the Holocene in relation to mean state changes

A consensus has not yet been reached when it comes to the long-term changes in ENSO diversity. Indeed, for models that simulate larger warming in the East Pacific, some studies show an increase of Eastern Pacific (EP) events, and a decrease in Central Pacific (CP) events, or the opposite. Similar apparent contradictions also emerge from analyses of the changes in EP versus CP El-Niño events in the Holocene. In this study, we consider the Holocene period as a means to study long-term El Niño variability in a context relatively close to the present. Indeed, the Holocene period allows studying the changes related to the long-term trend induced by the long-term evolution of the Earth’s orbit and seasonal evolution induced by the orbital forcing. We use two 6,000-year-long transient simulations of the IPSL model and two different indicators to characterize El Niño events. 

This study shows that we can have opposite results on the behavior of EP and CP events depending on the type of indicator used to characterize El Niño. We will discuss the reasons for these contrasting results, as seen in two previous studies. Moreover, we will test the extent to which the types of events are induced by changes in the tropical Pacific’s thermocline.